1. Field of the Invention
The invention relates generally to the analysis of underground earth formations, and, more particularly, to the determination of formation resistivity properties and hydrocarbon profiles of same.
2. Background Art
Knowledge of the resistivity properties of underground earth formations is fundamental in the evaluation and characterization of potential and existing hydrocarbon-bearing reservoirs. Resistivity measurements from within a borehole drilled through these formations have been made by a number of techniques employing the use of well-known tools or instruments, including induction, propagation, neutron, sonic, and laterolog (electrode) type devices.
The systematic measurement of properties or variations in a characteristic of the formations around a borehole and the recording of such measurements as a function of depth and/or time is commonly referred to as a “log” or “logging.” Well logs are typically recorded by inserting the various types of measurement tools into a borehole, moving the tools along the borehole, and recording the measurements made by the tools.
Conventional logging techniques include “wireline” logging and logging-while-drilling (LWD), also referred to as measurement-while-drilling (MWD). Wireline logging entails lowering the measuring tool into a drilled borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is suspended within the borehole. LWD/MWD entails attaching the tool disposed in a drill collar to a drilling assembly while a borehole is being drilled through the formations. [As used herein, LWD/MWD is intended to include the taking of measurements in an earth borehole, with the drill bit and at least some of the drill string in the borehole, during drilling, pausing, and/or tripping.]
In LWD/MWD operations, drilling fluid (mud) is used to lubricate and cool the drill bit and to carry borehole cuttings upwardly to the surface as the borehole is drilled. Drilling muds are generally either water based or oil based. Since 1990, synthetic-based muds have also been introduced in the industry.
The mud is also used in LWD operations to transmit measurement data to the surface using a pressure modulation telemetry system, which modulates pressure of the mud flowing through the interior of the drilling tool assembly. A much larger amount of well log data is typically stored in a recording device disposed in the logging tool, which is interrogated when the tool is retrieved from the borehole. A record of tool position in the borehole with respect to time is then correlated to the time/measurement record retrieved from the tool storage device to generate a conventional “well log” of measurements with respect to borehole depth.
Geologists and petrophysicists historically have found it necessary to visually analyze full well cores extracted from zones of interest to assess complex thinly laminated (also referred to as bedded) reservoirs and aid in the discovery and evaluation of hydrocarbon reservoirs. High-resolution “micro-resistivity” measurement techniques have also been developed over the years to contribute to the identification of hydrocarbons in low resistivity pay zones in wells drilled with water-based mud. One such conventional high-resolution logging tool is described in U.S. Pat. No. 6,191,588 (assigned to the present assignee) and in Cheung P., et al., Field Test Results of a New Oil-Base Mud Formation Imager Tool, SPWLA 2001, paper XX.
High-resolution measurements have also helped improve the estimation of reserves in such reservoirs. These techniques require that the mud be conductive, usually a mixture of salt water and weighting solid materials to control mud density. Latter measurement techniques, such as ones based on the tool described in the '588 patent, provide high-resolution images of the borehole wall surface. These techniques provide several high-resolution micro-resistivity measurements capable of detecting the presence of thin layers of the order of several tens of inches to a few inches. Such layers are much too thin to be accurately measured by standard resistivity logging tools since the layer thickness are much smaller than the vertical resolution of standard tools.
Most resistivity measurement tools have been designed to investigate on the order of about 1 to 3 feet [0.3-0.91 m] into the reservoir beyond the invaded zone. Such invaded zone is always present when the well is drilled with a water-base mud. However, more and more new wells are being drilled with oil-based mud containing chemical additives that build and leave a thin impermeable mudcake and usually prevent significant invasion into the permeable zones around the borehole.
Comparisons between full well cores and micro-resistivity logs or images have contributed to industry acceptance of this technology in evaluating the potential of the reservoir by simply “counting” the sand layers. In shaly sand formations, the shale layers are usually of low resistivity while the sand layers saturated with hydrocarbons will have a much higher resistivity. The same sand layers, depending on their grain size, are less resistive than the sand layers containing hydrocarbon and are often even less resistive than the shale layers. In turbiditic depositional environments, one can often find water-bearing sand layers with resistivity in the range of 0.1 to 0.5 ohm-meters, shale layers in the range of 0.4 to 5 ohm-meters, while hydrocarbon sand layers can reach a resistivity of several tens or hundred ohms-meter depending on their porosity and hydrocarbon content. Interlaced with these sand-shale layers, some tight layers with very high resistivity, often greater than thousands of ohms-meters, might also be present.
Evaluation of thinly laminated reservoirs is not a new problem in formation evaluation and interpretation. See, for example, U.S. Pat. Nos. 3,166,709, 5,461,562 (assigned to the present assignee). Historically there have been three methods used by the industry to evaluate thinly laminated reservoirs:                1. Sand Count: Counting the sand layers and cumulating the total sand length;        2. SHARP: Synergetic High Resolution Analysis and Reconstruction for Petrophysical Evaluation; and        3. Anisotropy measurement: Measurement of bulk anisotropic formation parameters.        
Sand Count. The simplest technique is referred to as “Sand Count.” It is a technique used in conjunction with conventional micro-resistivity devices. This method attempts to classify each detected layer into one of several classes. Whenever the micro-resistivity log value of a given layer is falling into the specific resistivity limits, that layer is assigned to a specific class. However the micro-log generally provided two micro-resistivity logs with different depths of investigation. In front of permeable beds, a separation caused by the mud-cake buildup would signify that a layer was permeable and should not be confused with a tight streak that would not cause any separation between the two micro-resistivity logs. The simplest interpretation model has a minimum of two classes, a “sand class” and a “shale class.” Four classes are often found necessary to perform a more accurate sand count in more complex environments.
SHARP. The SHARP method combines tool measurements with different vertical resolution. It takes advantage of one or several high resolution logs to define the precise location of the boundaries of each thin layer detected by the high-resolution logs. However the resistivity value estimated by the tool(s) is usually not the true resistivity of the layers. The micro-resistivity is usually lower than the true resistivity of the layers because of invasion, especially when the mud is conductive. The main concept behind SHARP is that the convolution of a high resolution log with a vertical response function, representing the vertical tool response of a low resolution log, should match the low resolution real log whenever the high resolution log represents the true value of each layer. The SHARP technique is further described in U.S. Pat. No. 5,461,562.
Anisotropy Measurement. An earth formation is anisotropic whenever its petrophysical properties are different in two distinct directions. For instance the resistivity of an earth formation is usually found to be higher when measured across the bedding planes and lower when it is measured parallel to the same bedding plane. Formation resistivity anisotropy is a characteristic of reservoirs that can complicate formation evaluation. Many reservoir rocks exhibit resistivity anisotropy, especially when saturated with oil. There are several mechanisms, which can produce this anisotropy, among which are very thin sand-shale laminations, depositional changes in clean sandstone, and wind-distributed sands (aeolian formations). See Rubin, D. M., Cross bedding, bedforms, and paleocurrents, SOCIETY OF ECONOMIC PALEONTOLOGISTS AND MINERALOGISTS, CONCEPTS IN SEDIMENTOLOGY AND PALEONTOLOGY, 1; Klein et al., The petrophysics of electrically anisotropic reservoirs, Transactions of the SPWLA THIRTY-SIXTH ANNUAL LOGGING SYMPOSIUM, Paris, France, Jun. 26-29, 1995, paper HH.
Formation anisotropy is not restricted to resistivity. Earth formations also exhibit permeability and acoustic anisotropy. Conventional propagation-type logging instruments are sensitive to anisotropy and can be used to estimate it in deviated wells or high relative dip situations. Recently, triaxial induction-type instruments have been introduced in the industry to make such estimates regardless of the hole deviation and value of relative dip. Combinations of existing resistivity logging instruments allow for the estimation of formation anisotropy. However this latter method is primarily limited to wells drilled with conductive muds.
Thus there remains a need for improved techniques for evaluating anisotropy profiles and hydrocarbon reserves of thinly laminated earth formations, particularly where the borehole is drilled with a substantially conductive fluid.